The present invention relates to an optical system for reading from and writing information on an information recording and/or reproducing medium such as an optical disk and, more particularly, to an optical head for projecting a laser beam onto an information recording and/or reproducing medium read from and write information on it.
In an optical system which reads out from or writes information on an information recording and/or reproducing medium, a laser beam is projected onto the information recording and/or reproducing medium (to be referred to as an optical disk hereinafter) through an optical head. The laser beam reflected from the optical disk is picked up by the optical head. In order to write information on the optical disk and read it out therefrom properly, the focal point of an objective lens of the optical head must be accurately placed on the light reflecting surface of the optical disk. In other words, the beam waist of the laser beam converged by the objective lens must be projected onto the optical disk. Various apparatuses have been developed to detect the focal point of the objective lens. However, none of them is satisfactory. For example, an apparatus which utilizes the difference between sizes of focused and unfocused beam spots on the optical disk is proposed wherein different patterns of the focused and unfocused beam spots are projected onto a photodetector and are detected by the photodetector to achieve proper focusing. Further, an apparatus for focusing a laser beam, which is provided with a lens system combining a convex lens and a cylindrical lens is disclosed in U.S. Pat. No. 4,079,247 of Bricot et al. In these apparatuses, if minuteness recess or projections are formed on the optical disk, a diffraction pattern is formed in the beam spot pattern on the photodetector, resulting in erroneous operation. Especially, in an optical disk which has a tracking guide to increase information recording capacity, a diffraction pattern of the tracking guide is formed in the beam spot pattern on the photodetector when the beam spot is formed on the tracking guide. As a result, the apparatus may be erroneously operated.
In the apparatus described above, the out-of-focus state of the objective lens is detected by changes in the size of the beam spot pattern on the photodetector or in the shape of the beam spot pattern. Another apparatus is proposed which detects the out-of-focus state of the objective lens by a location of a beam spot pattern formed on a photodetector. In this apparatus, laser beam for detecting the out-of-focus state of the objective lens is incident on the objective lens parallel to an optical axis of the objective lens and projected onto the optical disk therethrough, in addition to laser beams which are used for readout and writing of information. However, it has a drawback that an optical system becomes complex and the apparatus is manufactured at high cost, because the optical system having at least two optical paths for the laser beams is required. Other apparatuses are disclosed in Japanese Patent Disclosure Nos. 53-28,405, 49-31,128, and 53-10,407, respectively. In these apparatuses, laser beams for reading out and writing information are not travelled on an optical axis of an objective lens but are travelled in parallel to the axis thereof. However, in these apparatuses the laser beams cannot be sufficiently converged by the objective lens, and a sufficiently small beam spot cannot be formed on the optical disk. Further, since the laser beams pass through the outer peripheral portion of the objective lens and are projected onto the optical disk, the intensity of laser beams projected onto the optical disk is decreased by eclipse.
Further, an apparatus is dislosed in Japanese Patent Disclosure No. 53-118,103, in which a prism is arranged on an optical path of laser beams reflected by an optical disk. In this apparatus, the intensity of the laser beams may be attenuated when they pass through the prism, and unwanted diffraction may occur.